1. Field of the Invention
The invention concerns magnetrons and, more particularly, agile magnetrons ; namely magnetrons whose frequency can be tuned very rapidly.
Magnetrons such as this are generally made according to two types of techniques.
In the first technique, a system of fins is made to rotate at the end of the microwave circuit of the magnetron, and the presence of this system of fins modifies the tuning frequency. The frequency of the magnetron then varies according to the periodicity, which may or may not be regular, of passage of the fins driven by a rotating motor. These magnetrons are called "spin-tuned magnetrons".
In the second technique, the tuning is modified by a piston that moves linearly in a tuning cavity. The system is moved by a linear motor formed like a loudspeaker coil. These magnetrons are called "voice-coil magnetrons". A coil through which a variable current flows is placed in the gap of a magnet producing a field that is radial with respect to the coil.
In these two techniques, the (linear or rotary) motor has windings through which relatively high currents flow. These windings get heated up and the motor is cooled by thermal conduction through the mechanical supports and by thermal convection through the atmosphere surrounding the windings of the motor.
To resolve the problem of heat dissipation, it has been sought to optimize parameters such as the number of turns of the windings and the section of the wires forming the windings.
To obtain the benefit of high agility with respect to frequency, a low mass of windings must be used. This enables reducing the kinetic energy of the movable unit, hence the power needed to move it.
It is, therefore, necessary to reduce the length of the winding turns and/or their section, i.e. three parameters can be brought into play: the number of turns, the section of the winding and the diameter of the turns. However, if the number of turns is reduced, the electromagnetic force exerted on the winding is reduced. If the diameter of the turns is reduced, the same thing happens. And if the section of the wires of the winding is reduced, the electrical resistance of the winding is increased, hence the quantity of heat is increased.
There are, therefore, severe limits as regards the possibilities of choosing the number of turns, the section of the wires and the diameter, and it is seen that, to obtain a truly agile magnetron, there has to be a motor dissipating a high quantity of heat in a small volume.
Typical values computed are, for example, a diameter of 48 mm., resistance of 0.02 ohms, current density of 30 amperes per square millimeter of conductor section, heat dissipation of 50 watts, a coil area of about 15 square centimeters to dissipate the power and, consequently, a density of power to be removed of the order of 3 watts per square centimeter of the external surface of the coil.